1. Field
Exemplary embodiments of the present invention generally relate to data transmission over fiber optic networks. More particularly embodiments of the present invention relate to the detection of power transients in fiber optic networks.
2. Background
Current fiber-optic long-haul communication networks are predominantly comprised of point-to-point fiber-optic links. The data-modulated optical signals originated at one end and propagate through the fiber medium to the opposite end. While propagating through the fiber medium, the optical signals can suffer attenuation due to the scattering in the fiber medium, as well as losses in other components such as couplers, connectors and the like. To compensate for the loss, optical amplifiers can be placed at regular intervals along the fiber span, typically 40 to 100 km apart. A single fiber strand can carry many independent multiple optical signals (e.g., >100), each signal being differentiated by a slightly different wavelength (e.g., 0.4 nm separation). Accordingly, optical amplifiers amplify all the wavelengths simultaneously. As is known in the art, it is common for the optical amplifiers to be operated in a saturated mode having a fixed total optical power output, but variable gain.
More recently, optical communication networks have started to evolve away from simple point-to-point links. The first step was the introduction of fixed optical add-drop multiplexers (OADMs). The OADMs can be positioned at intermediate points along the fiber-optic link between the terminal ends, and provide the capability for adding or dropping individual wavelengths. This diversity of signal origination and termination points allows for more flexible and useful optical network architectures. A second evolutionary step was the addition of dynamic OADM capability, such that individual optical wavelength signals can be dynamically switched and rerouted between various fiber-optic links.
However, both of the above scenarios create a problem in controlling optical power in each wavelength. As previously mentioned, optical amplifiers are commonly operated such that they provide a fixed total output power (constant power mode), which is proportioned among the various wavelengths. This configuration provides an undesirable coupling mechanism among the optical wavelengths. Optical wavelength signals can appear and disappear in the fiber-optic link, either due to component failures and/or fiber cuts in the fixed OADM case, or due to active wavelength switching in the dynamic OADM case. As optical wavelength signals disappear, optical amplifiers operating in a constant power mode allocate the unused power to the remaining signals potentially causing a substantial increase in their power. Conversely, newly added optical wavelengths can cause substantial power drop in the already existing ones.
These optical power transients can be detrimental for several reasons. Optical power exceeding the receiver's dynamic range may cause loss of data on the low end and potential permanent component damage on the high end. For example, reduced optical wavelength power can cause signal to noise degradation and may result in a loss of data. Likewise, increased optical wavelength power can cause nonlinear signal distortions and noise and may result in a loss of data. Finally, optical power transients may disrupt seemingly unrelated parts of the network complicating alarm management and troubleshooting.
Additionally, the problem of channel loss exists. Channel loss can occur for several reasons in an optical network. For example, a fiber cut can occur between a first OADM and first optical line amplifier (OLA), which can cause a wavelength λ1 to be removed from the downstream optical amplifier chain. Poor connection quality, degradation or failure of components, and the like can also cause channel loss.
In one conventional system, the OLAs are held in constant power mode in an attempt to prevent channel/wavelength loss. In this system, the OLA pump power is held constant. Assuming a three channel system, if the second channel is lost, the available OLA output power is redistributed to the first and third channels, thereby proportionately increasing their power. However, fiber nonlinear effects may become detrimental to proper signal propagation, and data associated with these wavelengths may be lost.
In another conventional system, the OLAs are held in constant gain mode to prevent channel loss. OLA gain may be controlled via electronic feedback to the pump power, via optical feedback of a lasing wavelength, or other methods known in the art. In all cases, there is some finite error in gain in each amplifier associated with feedback circuit errors and response time, excited state absorption, and/or spectral hole burning, for example. Further, fiber nonlinear effects such as cross-channel Raman gain can substantially perturb the gain experienced by channels remaining in the system. Thus, remaining channel power can either increase or decrease in an unpredictable manner. If such deviations exceed the dynamic range of the system, data associated with these wavelengths will be lost.
As previously mentioned, a problem in conventional systems is loss increase due to fiber pinch or connector mismatching. A conventional system may attempt to limit loss by holding OLAs in constant power mode. The transmission system thus keeps channel output power constant for subsequent spans. However, the OLA immediately following the span where the loss increase occurred will likely experience a noticeable spectral tilt in overall gain. This may lead to data loss in the channels, particularly at the extreme ends of the spectrum.
In contrast, when the OLAs are held in constant gain mode the channel power in all subsequent spans will decrease by the same amount. This can lead to a substantial decrease in the optical signal to noise ratio. Accordingly, the data associated with these wavelengths may be lost.
Consequently, a need still exists for a system which provides a solution to the aforementioned data/signal loss problems in the conventional systems that operate in a constant power or constant gain mode.